Cellular and molecular mechanisms of chemical-induced ovarian toxicity
The ovary is the ovum-producing female reproductive organ composed of follicles at different stages of development. Chemicals which selectively damage large growing or antral follicles only temporarily interrupt reproductive function because these follicles can be replaced by recruitment from the greater pool of primordial follicles. However, chemicals that destroy oocytes contained in primordial follicles can cause permanent infertility and premature ovarian failure (early menopause in women), since once a primordial follicle is destroyed, it cannot be replaced. We hypothesized that ovotoxic chemicals can bind to DNA to induce DNA damage and that the ovarian cells activate DNA repair, cell cycle arrest and apoptosis to cause chemical-induced toxicity. Additionally, we investigated cell to cell communication as targets of ovotoxicants. Gap junction proteins are involved in folliculogenesis and we questioned whether an external factor such as ovotoxicant exposure and obesity alters gap junction proteins to cause follicle loss. To test this hypothesis, ovarian mRNA and protein expression profiles involved in DNA repair, cell cycle arrest, apoptosis and gap junction formation were quantified using qRT-PCR, western blotting or immunohistochemistry techniques after exposure to two different chemicals: Phosphoramide mustard (PM) and 7,12-dimethylbenz[a]anthracene (DMBA). In addition, we investigated the impact of PM and/or DMBA in three physiological paradigms: 1) spontaneously immortalized rat granulosa cells (SIGC); 2) In vitro neonatal ovarian culture system; 3) In vivo obese mouse model. Overall, our data demonstrates that the ovarian DNA damage responses are activated and up-regulated in both a dose- and time-dependent manner. Also, chemical exposure alters the gap junction protein expression in ovaries, interfering with inter-cellular communication, leading to follicular demise. Taken together, our discoveries detailed herein shed new mechanistic light on the events that precipitate ovarian follicle depletion leading to female infertility, and potential targets for prevention of chemical-induced infertility are described.